Managing grey zones of unreachable nodes in computer networks

ABSTRACT

In one embodiment, a node (e.g., a root-node) of a currently known directed acyclic graph (DAG) topology of a computer network can identify a sub-DAG of one or more nodes that are unreachable. The node can further determine a scope of the unreachable nodes of the sub-DAG and tunnel a redirected message to a reachable node of the DAG topology that is adjacent to at least one of the unreachable nodes of the sub-DAG. The redirected message may cause the reachable node to distribute the redirected message to one or more of the unreachable nodes of the sub-DAG based on the scope.

TECHNICAL FIELD

The present disclosure relates generally to computer networks, and, moreparticularly, to routing techniques for low power and lossy networks(LLNs).

BACKGROUND

Low power and Lossy Networks (LLNs), e.g., sensor networks, have amyriad of applications, such as Smart Grid and Smart Cities. One examplerouting protocol used for LLNs is a protocol called Routing Protocol forLLNs or “RPL,” which is a distance vector routing protocol that builds aDestination Oriented Directed Acyclic Graph (DODAG, or simply DAG) inaddition to a set of features to bound the control traffic, supportlocal (and slow) repair, etc. The RPL architecture provides a flexiblemethod by which each node performs DODAG discovery, construction, andmaintenance.

Various challenges are presented with LLNs, such as lossy links, lowbandwidth, battery operation, low memory and/or processing capability,etc. For example, communication in LLNs can be affected by variouscommunication conditions (e.g., temporal changes in interference,physical obstruction, propagation changes, etc.) that change over time.In addition, time scales of such temporal changes are variable and canrange between milliseconds (e.g. transmissions from other transceivers)to months (e.g. seasonal changes of outdoor environment). Accordingly,nodes within the LLN can be potentially unreachable due to the variouscommunication conditions. In particular, detecting these potentiallyunreachable nodes and re-routing messages destined for the unreachablenodes remains problematic for LLN networks.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein may be better understood by referring to thefollowing description in conjunction with the accompanying drawings inwhich like reference numerals indicate identically or functionallysimilar elements, of which:

FIG. 1 illustrates an example communication network;

FIG. 2 illustrates an example network device/node;

FIG. 3 illustrates an example view of a directed acyclic graph (DAG)topology of the communication network;

FIGS. 4-7 illustrate an example process for re-routing messages to reachunreachable nodes of a sub-DAG;

FIG. 8 illustrates an example simplified procedure for reachingunreachable nodes of a sub-DAG in a communication network; and

FIGS. 9-10 illustrate example sub-procedures for reaching unreachablenodes of the sub-DAG in the communication network.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

According to one or more embodiments of the disclosure, a node (e.g., aroot-node) of a currently known directed acyclic graph (DAG) topology ofa computer network can identify a sub-DAG of one or more nodes that areunreachable. The node can further determine a scope of the unreachablenodes of the sub-DAG and tunnel a redirected message to a reachable nodeof the DAG topology that is adjacent to at least one of the unreachablenodes of the sub-DAG. The redirected message may cause the reachablenode to distribute the redirected message to one or more of theunreachable nodes of the sub-DAG based on (e.g., limited by/to) thescope.

Description

A computer network is a geographically distributed collection of nodesinterconnected by communication links and segments for transporting databetween end nodes, such as personal computers and workstations, or otherdevices, such as sensors, etc. Many types of networks are available,ranging from local area networks (LANs) to wide area networks (WANs).LANs typically connect the nodes over dedicated private communicationslinks located in the same general physical location, such as a buildingor campus. WANs, on the other hand, typically connect geographicallydispersed nodes over long-distance communications links, such as commoncarrier telephone lines, optical lightpaths, synchronous opticalnetworks (SONET), synchronous digital hierarchy (SDH) links, orPowerline Communications (PLC) such as IEEE 61334, IEEE P1901.2, andothers. In addition, a Mobile Ad-Hoc Network (MANET) is a kind ofwireless ad-hoc network, which is generally considered aself-configuring network of mobile routes (and associated hosts)connected by wireless links, the union of which forms an arbitrarytopology.

Smart object networks, such as sensor networks, in particular, are aspecific type of network having spatially distributed autonomous devicessuch as sensors, actuators, etc., that cooperatively monitor physical orenvironmental conditions at different locations, such as, e.g.,energy/power consumption, resource consumption (e.g., water/gas/etc. foradvanced metering infrastructure or “AMI” applications) temperature,pressure, vibration, sound, radiation, motion, pollutants, etc. Othertypes of smart objects include actuators, e.g., responsible for turningon/off an engine or perform any other actions. Sensor networks, a typeof smart object network, are typically shared-media networks, such aswireless or PLC networks. That is, in addition to one or more sensors,each sensor device (node) in a sensor network may generally be equippedwith a radio transceiver or other communication port such as PLC, amicrocontroller, and an energy source, such as a battery. Often, smartobject networks are considered field area networks (FANs), neighborhoodarea networks (NANs), etc. Generally, size and cost constraints on smartobject nodes (e.g., sensors) result in corresponding constraints onresources such as energy, memory, computational speed and bandwidth.

FIG. 1 is a schematic block diagram of an example computer network 100illustratively comprising nodes/devices 200 (e.g., labeled as shown,“root,” “11,” “12,” . . . “45,” and described in FIG. 2 below)interconnected by various methods of communication. For instance, thelinks 105 may be wired links or shared media (e.g., wireless links, PLClinks, etc.) where certain nodes 200, such as, e.g., routers, sensors,computers, etc., may be in communication with other nodes 200, e.g.,based on distance, signal strength, current operational status,location, etc. Those skilled in the art will understand that any numberof nodes, devices, links, etc. may be used in the computer network, andthat the view shown herein is for simplicity. Also, those skilled in theart will further understand that while the network is shown in a certainorientation, particularly with a “root” node, the network 100 is merelyan example illustration that is not meant to limit the disclosure.

Data packets 140 (e.g., traffic and/or messages sent between thedevices/nodes) may be exchanged among the nodes/devices of the computernetwork 100 using predefined network communication protocols such as theTransmission Control Protocol/Internet Protocol (TCP/IP), User DatagramProtocol (UDP), Multi-Protocol Label Switching (MPLS), wirelessprotocols (e.g., IEEE Std. 802.15.4, WiFi, Bluetooth®, etc.), PLCprotocols, or other shared-media protocols where appropriate. In thiscontext, a protocol consists of a set of rules defining how the nodesinteract with each other.

FIG. 2 is a schematic block diagram of an example node/device 200 thatmay be used with one or more embodiments described herein, e.g., as anyof the nodes shown in FIG. 1 above. The device may comprise one or morenetwork interfaces 210 (e.g., wired, wireless, PLC, etc.), at least oneprocessor 220, and a memory 240 interconnected by a bus 250, as well asa power supply 260 (e.g., battery, plug-in, etc.).

The network interface(s) 210 contain the mechanical, electrical, andsignaling circuitry for communicating data over links 105 coupled to thenetwork 100. The network interfaces may be configured to transmit and/orreceive data using a variety of different communication protocols. Note,further, that the nodes may have two different types of networkconnections 210, e.g., wireless and wired/physical connections, and thatthe view herein is merely for illustration. Also, while the networkinterface 210 is shown separately from power supply 260, for PLC thenetwork interface 210 may communicate through the power supply 260, ormay be an integral component of the power supply. In some specificconfigurations the PLC signal may be coupled to the power line feedinginto the power supply.

The memory 240 comprises a plurality of storage locations that areaddressable by the processor 220 and the network interfaces 210 forstoring software programs and data structures associated with theembodiments described herein. Note that certain devices may have limitedmemory or no memory (e.g., no memory for storage other than forprograms/processes operating on the device and associated caches). Theprocessor 220 may comprise hardware elements or hardware logic adaptedto execute the software programs and manipulate one or more datastructures 245 such as routes/prefixes, etc. An operating system 242,portions of which are typically resident in memory 240 and executed bythe processor, functionally organizes the device by, inter alia,invoking operations in support of software processes and/or servicesexecuting on the device. These software processes and/or services maycomprise a routing process/services 244. It will be apparent to thoseskilled in the art that other processor and memory types, includingvarious computer-readable media, may be used to store and executeprogram instructions pertaining to the techniques described herein.Also, while the description illustrates various processes, it isexpressly contemplated that various processes may be embodied as modulesconfigured to operate in accordance with the techniques herein (e.g.,according to the functionality of a similar process).

Routing process (services) 244 contains computer executable instructionsexecuted by the processor 220 to perform functions provided by one ormore routing protocols, such as proactive or reactive routing protocolsas will be understood by those skilled in the art. These functions may,on capable devices, be configured to manage a network topology includingrouting/forwarding tables (a data structure 245) containing data used tomake routing/forwarding decisions. In particular, in proactive routing,connectivity is discovered and known prior to computing routes to anydestination in the network, e.g., link state routing such as OpenShortest Path First (OSPF), orIntermediate-System-to-Intermediate-System (ISIS), or Optimized LinkState Routing (OLSR). Reactive routing, on the other hand, discoversneighbors (i.e., does not have an a priori knowledge of networktopology), and in response to a needed route to a destination, sends aroute request into the network to determine which neighboring node maybe used to reach the desired destination. Example reactive routingprotocols may comprise Ad-hoc On-demand Distance Vector (AODV), DynamicSource Routing (DSR), DYnamic MANET On-demand Routing (DYMO), etc.Notably, on devices not capable or configured to store routing entries,routing process 244 may consist solely of providing mechanisms necessaryfor source routing techniques. That is, for source routing, otherdevices in the network can tell the less capable devices exactly whereto send the packets, and the less capable devices simply forward thepackets as directed.

Low power and Lossy Networks (LLNs) (e.g., certain sensor networks), maybe used in a myriad of applications such as for “Smart Grid” and “SmartCities.” A number of challenges in LLNs have been presented, such as:

-   1) Links are generally lossy, such that a Packet Delivery Rate/Ratio    (PDR) can dramatically vary due to various sources of interferences,    e.g., considerably affecting the bit error rate (BER);-   2) Links are generally low bandwidth, such that control plane    traffic must generally be bounded and negligible compared to the low    rate data traffic;-   3) There are a number of use cases that require specifying a set of    link and node metrics, some of them being dynamic, thus requiring    specific smoothing functions to avoid routing instability,    considerably draining bandwidth and energy;-   4) Constraint-routing may be required by some applications, e.g., to    establish routing paths that will avoid non-encrypted links, nodes    running low on energy, etc.;-   5) Scale of the networks may become very large, e.g., on the order    of several thousands to millions of nodes; and-   6) Nodes may be constrained with a low memory, a reduced processing    capability, a low power supply (e.g., battery).

In other words, LLNs are a class of network in which both the routersand their interconnect are constrained: LLN routers typically operatewith constraints, e.g., processing power, memory, and/or energy(battery), and their interconnects are characterized by, illustratively,high loss rates, low data rates, and/or instability. LLNs are comprisedof anything from a few dozen and up to thousands or even millions of LLNrouters, and support point-to-point traffic (between devices inside theLLN), point-to-multipoint traffic (from a central control point to asubset of devices inside the LLN) and multipoint-to-point traffic (fromdevices inside the LLN towards a central control point).

An example implementation of LLNs is an “Internet of Things” network.Loosely, the term “Internet of Things” or “IoT” may be used by those inthe art to refer to uniquely identifiable objects (things) and theirvirtual representations in a network-based architecture. In particular,the next frontier in the evolution of the Internet is the ability toconnect more than just computers and communications devices, but ratherthe ability to connect “objects” in general, such as lights, appliances,vehicles, HVAC (heating, ventilating, and air-conditioning), windows andwindow shades and blinds, doors, locks, etc. The “Internet of Things”thus generally refers to the interconnection of objects (e.g., smartobjects), such as sensors and actuators, over a computer network (e.g.,IP), which may be the Public Internet or a private network. Such deviceshave been used in the industry for decades, usually in the form ofnon-IP or proprietary protocols that are connected to IP networks by wayof protocol translation gateways. With the emergence of a myriad ofapplications, such as the smart grid, smart cities, and building andindustrial automation, and cars (e.g., that can interconnect millions ofobjects for sensing things like power quality, tire pressure, andtemperature and that can actuate engines and lights), it has been of theutmost importance to extend the IP protocol suite for these networks.

An example protocol specified in an Internet Engineering Task Force(IETF) Proposed Standard, Request for Comment (RFC) 6550, entitled “RPL:IPv6 Routing Protocol for Low Power and Lossy Networks” by Winter, etal. (March 2012), provides a mechanism that supports multipoint-to-point(MP2P) traffic from devices inside the LLN towards a central controlpoint (e.g., LLN Border Routers (LBRs) or “root nodes/devices”generally), as well as point-to-multipoint (P2MP) traffic from thecentral control point to the devices inside the LLN (and alsopoint-to-point, or “P2P” traffic). RPL (pronounced “ripple”) maygenerally be described as a distance vector routing protocol that buildsa Directed Acyclic Graph (DAG) for use in routing traffic/packets 140,in addition to defining a set of features to bound the control traffic,support repair, etc. Notably, as may be appreciated by those skilled inthe art, RPL also supports the concept of Multi-Topology-Routing (MTR),whereby multiple DAGs can be built to carry traffic according toindividual requirements.

A DAG is a directed graph having the property that all edges (and/orvertices) are oriented in such a way that no cycles (loops) are supposedto exist. All edges are contained in paths oriented toward andterminating at one or more root nodes (e.g., “clusterheads or “sinks”),often to interconnect the devices of the DAG with a largerinfrastructure, such as the Internet, a wide area network, or otherdomain. In addition, a Destination Oriented DAG (DODAG) is a DAG rootedat a single destination, i.e., at a single DAG root with no outgoingedges. A “parent” of a particular node within a DAG is an immediatesuccessor of the particular node on a path towards the DAG root, suchthat the parent has a lower “rank” than the particular node itself,where the rank of a node identifies the node's position with respect toa DAG root (e.g., the farther away a node is from a root, the higher isthe rank of that node). Further, in certain embodiments, a sibling of anode within a DAG may be defined as any neighboring node which islocated at the same rank within a DAG. Note that siblings do notnecessarily share a common parent, and routes between siblings aregenerally not part of a DAG since there is no forward progress (theirrank is the same). Note also that a tree is a kind of DAG, where eachdevice/node in the DAG generally has one parent or one preferred parent.

DAGs may generally be built (e.g., by DAG process 246) based on anObjective Function (OF). The role of the Objective Function is generallyto specify rules on how to build the DAG (e.g. number of parents, backupparents, etc.). In addition, one or more metrics/constraints may beadvertised by the routing protocol to optimize the DAG against. Also,the routing protocol allows for including an optional set of constraintsto compute a constrained path, such as if a link or a node does notsatisfy a required constraint, it is “pruned” from the candidate listwhen computing the best path. (Alternatively, the constraints andmetrics may be separated from the OF.) Additionally, the routingprotocol may include a “goal” that defines a host or set of hosts, suchas a host serving as a data collection point, or a gateway providingconnectivity to an external infrastructure, where a DAG's primaryobjective is to have the devices within the DAG be able to reach thegoal. In the case where a node is unable to comply with an objectivefunction or does not understand or support the advertised metric, it maybe configured to join a DAG as a leaf node. As used herein, the variousmetrics, constraints, policies, etc., are considered “DAG parameters.”

Illustratively, example metrics used to select paths (e.g., preferredparents) may comprise cost, delay, latency, bandwidth, expectedtransmission count (ETX), etc., while example constraints that may beplaced on the route selection may comprise various reliabilitythresholds, restrictions on battery operation, multipath diversity,bandwidth requirements, transmission types (e.g., wired, wireless,etc.). The OF may provide rules defining the load balancingrequirements, such as a number of selected parents (e.g., single parenttrees or multi-parent DAGs). Notably, an example for how routing metricsand constraints may be obtained may be found in an IETF Internet Draft,entitled “Routing Metrics used for Path Calculation in Low Power andLossy Networks” <draft-ietf-roll-routing-metrics-19> by Vasseur, et al.(Mar. 1, 2011 version). Further, an example OF (e.g., a default OF) maybe found in an IETF Internet Draft, entitled “RPL Objective Function 0”RFC 6553 by Thubert (March, 2012 version) and “The Minimum RankObjective Function with Hysteresis”<draft-ietf-roll-minrank-hysteresis-of-04> by O. Gnawali et al. (May 17,2011 version).

Building a DAG may utilize a discovery mechanism to build a logicalrepresentation of the network, and route dissemination to establishstate within the network so that routers know how to forward packetstoward their ultimate destination. Note that a “router” refers to adevice that can forward as well as generate traffic, while a “host”refers to a device that can generate but does not forward traffic. Also,a “leaf” may be used to generally describe a non-router that isconnected to a DAG by one or more routers, but cannot itself forwardtraffic received on the DAG to another router on the DAG. Controlmessages may be transmitted among the devices within the network fordiscovery and route dissemination when building a DAG.

According to the illustrative RPL protocol, a DODAG Information Object(DIO) is a type of DAG discovery message that carries information thatallows a node to discover a RPL Instance, learn its configurationparameters, select a DODAG parent set, and maintain the upward routingtopology. In addition, a Destination Advertisement Object (DAO) is atype of DAG discovery reply message that conveys destination informationupwards along the DODAG so that a DODAG root (and other intermediatenodes) can provision downward routes. A DAO message includes prefixinformation to identify destinations, a capability to record routes insupport of source routing, and information to determine the freshness ofa particular advertisement. Notably, “upward” or “up” paths are routesthat lead in the direction from leaf nodes towards DAG roots, e.g.,following the orientation of the edges within the DAG. Conversely,“downward” or “down” paths are routes that lead in the direction fromDAG roots towards leaf nodes, e.g., generally going in the oppositedirection to the upward messages within the DAG.

Generally, a DAG discovery request (e.g., DIO) message is transmittedfrom the root device(s) of the DAG downward toward the leaves, informingeach successive receiving device how to reach the root device (that is,from where the request is received is generally the direction of theroot). Accordingly, a DAG is created in the upward direction toward theroot device. The DAG discovery reply (e.g., DAO) may then be returnedfrom the leaves to the root device(s) (unless unnecessary, such as forUP flows only), informing each successive receiving device in the otherdirection how to reach the leaves for downward routes. Nodes that arecapable of maintaining routing state may aggregate routes from DAOmessages that they receive before transmitting a DAO message. Nodes thatare not capable of maintaining routing state, however, may attach anext-hop parent address. The DAO message is then sent directly to theDODAG root that can in turn build the topology and locally computedownward routes to all nodes in the DODAG. Such nodes are then reachableusing source routing techniques over regions of the DAG that areincapable of storing downward routing state. In addition, RPL alsospecifies a message called the DIS (DODAG Information Solicitation)message that is sent under specific circumstances so as to discover DAGneighbors and join a DAG or restore connectivity.

FIG. 3 illustrates an example DAG 300 that may be created, e.g., throughthe techniques described above, within network 100 of FIG. 1. Forinstance, certain links 105 may be selected for each node to communicatewith a particular parent (and thus, in the reverse, to communicate witha child, if one exists). These selected links form the DAG 310 (shown asthicker lines), which extends from the root node toward one or more leafnodes (nodes without children). Traffic/packets 140 may then traverseDAG 410 in either the upward direction toward the root or downwardtoward the leaf nodes.

As noted above, nodes within the LLN can be potentially unreachable dueto the various communication conditions, and detecting potentiallyunreachable nodes and re-routing messages destined for the unreachablenodes has been generally problematic in LLN networks. For example, nodesemploying the RPL protocol build and maintain DAG routing topology viathe DIO messages and the DAO messages, as described above. DAO messagesare particularly important to maintain downward routes, and loss of DAOmessages can prevent a DAG root node from delivering messages to allnodes within the sub-DAG. That is, downward routes are typically notupdated until DAO messages propagate toward the DAG root node.Accordingly, loss of DAO messages, such as due to nodes that becomeunreachable, can result in a loss of additional messages that are routedaccording to an outdated DAG topology.

The techniques herein provide a mechanism that maintains and updatesconnectivity for unreachable nodes (e.g., when DAO messages are lost).In particular, as described herein, the techniques detect and identifythe unreachable nodes (e.g., “grey zones”) of a sub-DAG (e.g., via ICMP“destination unreachable” packets), determine a scope of the unreachablenodes, and determine an alternate route toward the unreachable nodes(e.g., via alternate paths). Once determined, the techniques provide fortunneling a redirected message to an adjacent node to at least one ofthe unreachable nodes of the sub-DAG. The redirected message causes thereachable node to distribute the redirected message (e.g., multicast,broadcast) to one or more of unreachable nodes of the sub-DAG. Notably,the redirected message can include various time-to-live (TTL) parametersto further determine a scope of the unreachable nodes of the sub-DAG.Each unreachable node that receives the redirected message can furtherinitiate a new DAO, which allows the DAG root to repair and update theDAG routing topology and eliminate unreachable nodes.

Illustratively, the techniques described herein may be performed byhardware, software, and/or firmware, such as in accordance with DAGprocess 246, which may contain computer executable instructions executedby the processor 220 to perform functions relating to the techniquesdescribed herein, e.g., in conjunction with routing process 244. Forexample, the techniques herein may be treated as extensions toconventional protocols, such as the RPL protocol, and as such, would beprocessed by similar components understood in the art that execute theRPL protocol, accordingly.

FIGS. 4-7 illustrate a progression of an example re-routing process toredirect messages to reach unreachable nodes of a sub-DAG. Inparticular, FIG. 4 illustrates a sub-DAG of unreachable nodes (e.g. “agrey zone”). Grey zones (or areas) exist where there is a lack of properrouting information to properly forward messages (e.g., data packets)even though there may be a feasible underlying path. For example, asdiscussed above, a grey zone may appear if a DAO message is lost when anode attempts to report a new DAG parent due to a damaged link or pathto an old DAG parent. In this instance, the DAG root may continue to usethe broken route for the node and any node in its sub-DAG. As shown, aDAO message from node 34 is lost, which may cause nodes 34, 44, and 45to become unreachable since the DAG root will continue to use link 24-34to deliver messages. Although connectivity to 34 and 45 may be restoredif the lost DAO message with the updated parent route is eventuallydelivered to the DAG root, the network must make a tradeoff betweensending DAO messages more frequently and incurring to much controlmessage overhead. Importantly, nodes 34, 44, and 45 are unreachable dueto a lost DAO message, however; nodes 34 and 45 have links to otherneighbor or adjacent nodes (e.g., nodes 33 and 35). Accordingly, nodes34 and 45 may become reachable if the DAG root uses a different route orpath.

Still referring to FIG. 4, one or more of the nodes of a sub-DAG (e.g.,nodes 34, 44, and 45) may be identified as unreachable. For example, aparent node (e.g., node 24) may generate a number of error messages(e.g., internet message control protocol (ICMP) destination unreachablemessages, etc.) for messages destined for a particular child-node(whether independent messages or retransmissions of messages) andreturns the error messages to the DAG root (or other source of themessage).

In one embodiment, the DAG root may buffer each packet for a limitedperiod of time for retransmission when ICMP errors are received. Inanother embodiment, the DAG root relies on the source of the message toimplement its own retry mechanism. In this instance, the DAG root maydetermine retransmitted packets by performing Deep Packet Inspection(DPI) or by caching identifiers (e.g. hashes) of the packets. The DAGroot may then compare the number of errors/retransmissions against athreshold count. Once the threshold count is exceeded, the child-node(e.g., the next hop node of node 24), including any sub-nodes connectedthereto, can be identified as unreachable. As shown, the unreachablenodes illustratively comprise a sub-DAG rooted at node 34 and includingnodes 44 and 45. In this fashion, nodes 44 and 45 share a sub-root orparent node 34 and the scope of the unreachable nodes can be determinedbased on the DAG topology of the sub-root node 34. In some embodiments,the DAG root can solicit additional information via active probing forparticular unreachable nodes, such as determining the scope ofunreachable nodes via sending connectivity verification messages (e.g.,ICMP echo messages) to one or more nodes within the sub-DAG.

According to the techniques described herein, the DAG root (or otherconfigured device) may attempt to reach nodes within a grey zone ofunreachable nodes by sending messages toward nodes “close to” (e.g.,adjacent to) the grey zone, in hopes that the adjacent nodes may provideconnectivity to one or more grey zone nodes without relying on theoutdated routing topology. In particular, as shown in FIG. 5, messagesor packets destined for the one or more of the unreachable nodes of thesub-DAG can be redirected via tunneling, that is, encapsulating aredirected message and tunneling the redirected message (e.g., a controlmessage or a data message) destined for one or more of the unreachablenodes to an adjacent node (e.g., node 33). (Note that “adjacent” neednot limit the selection of the node to one linked to a grey zone node,but to one that may reach the grey zone via a limited number of hops.)

The adjacent node 33 may thus receive the redirected message anddistributes or transmits (e.g., multicast, broadcast, etc.) theredirected message toward the one or more unreachable nodes—here theredirected message may reach sub-root node 34 through the distribution.Note that through the nature of a distributed message, other nodes mayalso receive and further distribute the message, as will be understoodby those skilled in the art. According to one or more embodimentsherein, adjacent node 33 can distribute the message in a limited fashionaccording to the determined scope of the unreachable nodes of thesub-DAG (the grey zone). For example, adjacent node 33 may be instructedto limit the number of next hops for the redirected message via atime-to-live (TTL) parameter within the message that was tunneled by theroot node.

FIG. 6 illustrates an unreachable sub-root node (i.e., a root of asub-DAG, e.g., node 34) receiving the redirected message anddistributing the redirected message to respective sub-nodes—nodes 44 and45. When the redirected message reaches an intended destination, thedestination node consumes the packet and refreshes or updates arespective DAO. The updated DAO is propagated to the DAG root, whichthen updates the DAG topology, thus removing the unreachable node fromthe grey zone. For example, FIG. 7 illustrates an updated DAG topology(e.g., network 700) showing node 33 connecting to node 34 via an updatedDAG route (e.g., where link 34-24 is unavailable).

Collectively FIGS. 4-7 illustrate identifying one or more unreachablenodes of a sub-DAG having a sub-DAG root node 34, identifying anadjacent node 33 of at least one of the unreachable nodes, encapsulatinga redirected message in a tunnel destined for an unreachable node,receiving the redirected message at the adjacent node and transmittingthe redirected message to all reachable nodes relative to the adjacentnode (including the previously unreachable node 34). In turn, sub-DAGroot node 34 receives the redirected message and distributes theredirected message to intended destination(s)—nodes 44 and 45.

As discussed above, the redirected message can include a time-to-liveparameter, which determines a number of next hop transmissions, that is,how far a distributed message will reach. In the event the limited scopeof the grey zone was not sufficient to reach the intended target (e.g.,the TTL was too small), then additional redirected messages having anincreased scope (e.g., a larger TTL parameter) can be sent. For example,as a redirected message fails to reach the intended destination, errormessages are generated. These error messages can indicate a scope andsize of the unreachable nodes. In some embodiments, the TTL preventsconsumption of network resources and provides granular control. Forexample, if the TTL parameter of a redirected message increases beyond athreshold, the DAG root can send subsequent redirected messages to adifferent adjacent node in an attempt to reach a destination node.

Moreover, in some embodiments, an alternate parent route within the DAGtopology can be determined for the unreachable nodes according to an RPLalternate route. That is, the DAG topology can be updated to restorereachability to nodes having alternate routes prior to (or in additionto) redirected messages. DAO messages can include a node's backupparent(s) in addition to its primary parent. Accordingly, the DAG Rootmay already know of alternate paths to reach a device. In this fashion,the DAG Root can update its routing table by changing node 45'spreferred parent to node 35. Note that by addressing nodes within thesub-DAG, the DAG Root can proactively restore connectivity to certain(or even all) nodes before even attempting to send a message to thosenodes and reacting to destination unreachable messages. Any node thatthe DAG Root can proactively restore connectivity to may thus be removedfrom the grey zone, potentially limiting the scope of distributedmessages.

FIG. 8 illustrates an example simplified procedure 800 for reachingunreachable nodes of a sub-DAG in a communication network (e.g., a greyzone). The procedure 800 starts at step 805, and continues to step 810,where, as described in greater detail above, a DAG root node canidentify a sub-DAG of one or more unreachable nodes. Upon identifyingthe unreachable nodes of a grey zone, in step 815, a scope of theunreachable nodes of the sub-DAG can be determined. For example, thescope can be determined based on routing control messages, connectivityverification messages, a sub-DAG root topology, etc.

Once the scope of the grey zone is determined, then in step 820, theDAG-root device may tunnel a redirected message to a reachable node ofthe DAG topology that is adjacent to at least one of the unreachablenodes of the sub-DAG. For example, as discussed above, the redirectedmessage can be encapsulated and tunneled to an adjacent or neighboringnode. The reachable adjacent node may then distribute (e.g., multicasts,broadcasts, etc.) the redirected message to one or more of theunreachable nodes of the sub-DAG in step 825 in hopes of reaching theintended destination nodes. As discussed above, the redirected messagecan be distributed in a limited or controlled fashion, which can bebased on, for example, the scope of the sub-DAG (e.g., via a TTL fieldwithin the message, etc.). When the redirected message is received at anintended destination, the intended destination may then provide updatedrouting information, such as an updated DAO as described above.

Accordingly, in step 830, the DAG root device can receive an updatedrouting control message (e.g., a DAO), from one or more particular nodes(e.g., the intended destination node(s)), of the unreachable sub-DAG. Inresponse, in step 835 the DAG-root can update the DAG topology for thenetwork (e.g., based on the received updated routing control message).The procedure 800 illustratively ends at step 840.

In addition, FIG. 9 illustrates an example simplified sub-procedure 900for reaching unreachable nodes of a grey zone, particularly with regardto an underestimated grey zone scope. In particular, sub-procedure 900starts at step 905, and continues to step 910, where, as described ingreater detail above, a DAG root node can send a redirected messagetoward one or more unreachable nodes of the sub-DAG by redirecting themessage to a reachable adjacent node for controlled or limiteddistribution using a specified TTL parameter value. Next, as discussedabove, in step 915 the DAG root node and/or the adjacent node determinesthat the redirected message cannot reach an intended destination (e.g.,a particular unreachable node). For example, the redirected message mayinclude a local timer parameter that starts a timer at the adjacentnode, expiration of which may result in a determination that theredirected message cannot reach the intended destination. Next, in step920, the TTL parameter of the redirected message is increased, yieldinga second redirected message, which, in step 925 may be tunneled to thereachable adjacent node for distribution (unless, that is, the TTL isincreased by the adjacent node itself). In some embodiments, the localtimer parameter (specified in the redirected message) can also beincreased. Notably, steps 915-925 can be iteratively repeated until theredirected message reaches an intended destination shown in step 930.Alternatively, a new adjacent node can be selected once the TTLparameter is increased beyond a threshold. Procedure 900 may thenillustratively end in step 945.

Moreover, FIG. 10 illustrates another example simplified sub-procedure1000 for reaching unreachable nodes of a grey zone, particularly withregard to decreasing the scope of a grey zone through known routealternates. In particular, sub-procedure 1000 starts at step 1005, andcontinues to step 1010, where, as described in greater detail above, aDAG root can determine an alternate parent node route (e.g., a RPLalternate parent node route) within the DAG topology for the one or moreunreachable nodes. Next, in step 1015, each unreachable node that can bereached via the alternate parent node is removed from the unreachablesub-DAG (from the grey zone), and in step 1020, the DAG root node canupdate the DAG topology based on the alternate parent node route.Procedure 1000 may then subsequently end in step 1020, which may furtherreturn to step 810 of FIG. 8 with a reduced number of unreachable nodeswithin the grey zone, accordingly.

It should be noted that while certain steps within procedures 800-1000may be optional as described above, the steps shown in FIGS. 8-10 aremerely examples for illustration, and certain other steps may beincluded or excluded as desired. Further, while a particular order ofthe steps is shown, this ordering is merely illustrative, and anysuitable arrangement of the steps may be utilized without departing fromthe scope of the embodiments herein. Moreover, while procedures 800-1000are described separately, certain steps from each procedure may beincorporated into each other procedure, and the procedures are not meantto be mutually exclusive.

The techniques described herein, therefore, address the issue of greyzones within computer networks, such as due to the loss of DAO messageswhich can lead to unreachable nodes based on out-of-date routinginformation (e.g., “black holes”). In particular, the techniques hereinprovide for granular control to recover such unreachable nodes andincrease overall network availability, without requiring additionalcontrol plane overhead.

While there have been shown and described illustrative embodiments thataddress the issue of grey zones within computer networks, it is to beunderstood that various other adaptations and modifications may be madewithin the spirit and scope of the embodiments herein. For example, theembodiments have been shown and described herein with relation to DAGnetworks. However, the embodiments in their broader sense are not aslimited, and may, in fact, be used with other types of networks and/orprotocols (e.g., wireless). In addition, while certain protocols areshown, such as RPL, other suitable protocols may be used, accordingly.Also, while the techniques generally describe initiation anddeterminations by a DAG root node, various other nodes may also be usedto provide intelligence to the network functions described herein (e.g.,non-root nodes, network management nodes, etc.).

The foregoing description has been directed to specific embodiments. Itwill be apparent, however, that other variations and modifications maybe made to the described embodiments, with the attainment of some or allof their advantages. For instance, it is expressly contemplated that thecomponents and/or elements described herein can be implemented assoftware being stored on a tangible (non-transitory) computer-readablemedium (e.g., disks/CDs/RAM/EEPROM/etc.) having program instructionsexecuting on a computer, hardware, firmware, or a combination thereof.Accordingly this description is to be taken only by way of example andnot to otherwise limit the scope of the embodiments herein. Therefore,it is the object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of theembodiments herein.

1. A method, comprising: identifying, within a currently known directedacyclic graph (DAG) topology of a computer network, a sub-DAG of one ormore nodes that are unreachable; determining a scope of the unreachablenodes of the sub-DAG; and tunneling a redirected message to a reachablenode of the DAG topology that is adjacent to at least one of theunreachable nodes of the sub-DAG, the redirected message causing thereachable node to distribute the redirected message to one or more ofthe unreachable nodes of the sub-DAG based on the scope.
 2. The methodof claim 1, further comprising: receiving an updated routing controlmessage from one or more particular nodes of the one or more unreachablenodes that receives the redirected message; and updating the DAGtopology of the computer network based on the received updated routingcontrol message.
 3. The method of claim 1, wherein the redirectedmessage is one of either a control message or a data message.
 4. Themethod of claim 1, further comprising: determining an alternate parentnode route within the DAG topology for one or more particular nodes ofthe unreachable nodes; removing each unreachable node that can bereached via the alternate patent node route from the sub-DAG; andupdating the DAG topology to reflect the alternate parent node route foreach corresponding unreachable node.
 5. The method of claim 1, whereinthe redirected message has a time-to-live (TTL), the method furthercomprising: determining that the redirected message cannot reach aparticular unreachable node of the one or more unreachable nodes;increasing the TTL of the redirected message yielding a secondredirected message; and tunneling the second redirected message to thereachable node to cause the reach-able node to distribute the redirectedmessage to the particular unreachable node with the increased TTL. 6.The method of claim 1, further comprising: determining that theredirected message cannot reach a particular unreachable node of the oneor more unreachable nodes; increasing the TTL of the redirected message;comparing the increased TTL of the redirected message against athreshold; and tunneling the redirected message having the increased TTLto the reachable node when the increased TTL is less than the threshold.7. The method of claim 6, wherein the redirected message having theincreased TTL is a probe message, the method further comprising:tunneling the redirected message to a different reachable node of theDAG topology that is adjacent to the at least one of the unreachablenodes of the sub-DAG when the redirected message having the increasedTTL is greater than the threshold.
 8. The method of claim 1, wherein theredirected message causes the reachable node to distribute theredirected message via at least one of broadcasting or multicasting. 9.The method of claim 1, wherein the one or more nodes of the sub-DAG arenodes of the DAG topology sharing a same sub-root node that isunreachable by a root node of the DAG topology.
 10. The method of claim1, wherein determining the scope of the unreachable nodes of the sub-DAGfurther comprises: correlating one or more received destinationunreachable messages for the one or more unreachable nodes of thesub-DAG.
 11. The method of claim 10, wherein the destination unreachablemessages are internet control message protocol (ICMP) destinationunreachable messages.
 12. The method of claim 1, wherein determining thescope of the unreachable nodes of the sub-DAG further comprises: sendingconnectivity verification messages to one or more nodes within thevicinity of the sub-DAG to determine which of the one or more nodes areunreachable.
 13. The method of claim 1, wherein determining the scope ofthe unreachable nodes of the sub-DAG is based on knowledge of the DAGtopology of a sub-root node of the sub-DAG.
 14. The method of claim 1,wherein the identifying a sub-DAG of the one or more nodes that areunreachable is in response to receiving a particular number ofretransmitted messages destined to one or more of the unreachable nodesof the sub-DAG.
 15. An apparatus, comprising: one or more networkinterfaces to communicate within a computer network; a processor coupledto the network interfaces and adapted to execute one or more processes;and a memory configured to store a process executable by the processor,the process when executed operable to: identify, within a currentlyknown directed acyclic graph (DAG) topology of a computer network, asub-DAG of one or more nodes that are unreachable; determine a scope ofthe unreachable nodes of the sub-DAG; and tunnel a redirected message toa reachable node of the DAG topology that is adjacent to at least one ofthe unreachable nodes of the sub-DAG, the redirected message causing thereachable node to distribute the redirected message to one or more ofthe unreachable nodes of the sub-DAG based on the scope.
 16. Theapparatus as in claim 15, wherein the process when executed is furtheroperable to: receive an updated routing control message from one or moreparticular nodes of the one or more unreachable nodes that receives theredirected message; and update the DAG topology of the computer networkbased on the received updated routing control message.
 17. The apparatusas in claim 15, wherein the redirected message is one of either acontrol message or a data message.
 18. The apparatus as in claim 15,wherein the process when executed is further operable to: determine analternate parent node route within the DAG topology for one or moreparticular nodes of the unreachable nodes; remove each unreachable nodethat can be reached via the alternate patent node route from thesub-DAG; and update the DAG topology to reflect the alternate parentnode route for each corresponding unreachable node.
 19. The apparatus asin claim 15, wherein the process when executed is further operable to:determine that the redirected message cannot reach a particularunreachable node of the one or more unreachable nodes; increase the TTLof the redirected message yielding a second redirected message; andtunnel the second redirected message to the reachable node to cause thereachable node to distribute the redirected message to the particularunreachable node with the increased TTL.
 20. A tangible, non-transitory,computer-readable media having software encoded thereon, the softwarewhen executed by a processor operable to: identify, within a currentlyknown directed acyclic graph (DAG) topology of a computer network, asub-DAG of one or more nodes that are unreachable; determine a scope ofthe unreachable nodes of the sub-DAG; and tunnel a redirected message toa reachable node of the DAG topology that is adjacent to at least one ofthe unreachable nodes of the sub-DAG, the redirected message causes thereachable node to distribute the redirected message to one or more ofthe unreachable nodes of the sub-DAG based on the scope.